1. Field of the Invention
The present invention relates to the field of air permeable material films. More particularly, the present invention relates to a reusable air permeable packaging film with improved air and moisture vapor permeability, which is particularly suited for microwave heating.
2. Description of the Prior Art
A wide variety of air and moisture vapor permeable materials have been developed for different purposes. For example, these materials can be used in filtration and separation. In U.S. Pat. No. 5,928,582, for example, there is disclosed a method of forming a microporous membrane that uses a process of ultraviolet irradiation to form microsphereulites, followed by a thermally-induced phase separation, yielding microporous membranes that have improved flow and mechanical properties. In U.S. Pat. No. 5,865,926, Wu et al. disclose a method of making a cloth-like microporous laminate of a non-woven fibrous web and thermoplastic film having air and moisture vapor permeability with liquid-barrier properties.
Other manufacturing processes for production of relevant microporous films are known in U.S. Pat. Nos. 3,378,507; 3,310,505; 3,607,793; 3,812,224; 4,247,498 and 4,466,931. For example, in U.S. Pat. No. 4,350,655, Hoge teaches a process for manufacturing a highly porous thermoplastic film formed by cold drawing a film of a synthetic thermoplastic orientative polymer, such as high-density polyethylene, and mixed with coated inorganic filler. The highly porous thermoplastic film is produced by first casting a film of a blend of the polymer-coated inorganic filler mixture, cooling the film to a temperature of 70° C. and cold stretching the film mono-axially or bi-axially to develop the desired void volume and surface ruptures per unit area, thereby obtaining a resin content (by weight) per cubic centimeter of final product of about 0.18 to about 0.32 gm/cc.
The coated inert inorganic filler and the molten polymer are blended together to form a homogeneous mixture through a suitable mixing extruder. The molten mixture is extruded through a die with an opening from 0.006 inches to 0.010 inches in size. The blend is cold stretched mono-axially or bi-axially, preferably in a station provided with a set of grooved rollers. The groove pattern of the rolls is generally of a sinosoidal wave pattern, wherein the film is stretched in a manner to affect uniform stretching between contact points of the material to produce a material of larger dimension in the stretching direction.
Disadvantageously, however, the manufacturing processes of microporous film products according to the prior art methods are too complicated and too expensive to be generally accepted. Furthermore, many operating factors, such as processing temperature, stretching ratios, film thickness, starting materials etc., affect the microporous size of the final products, and thus result in variations of the quality of the microporous film products. In addition, tile filler added to the microporous film products according to the prior art methods is a source of environmental pollution. Furthermore, most of the film products according to the prior art methods are opaque due to the multiple phases of the film products that result from the addition of fillers.
When food has, heretofore, been cooked at home, the food has been wrapped in a wrapping film for home use, or was packed and sealed in an air or moisture impermeable bag. This is heated in a microwave oven. Moisture contained in the food evaporates, and the bag is thus burst due to build up internal pressure. Moreover, when the film products according to the prior art methods are used to form a food-packaging bag, some of the fillers may contaminate the food within the bag, which results in unpleasant odors. Another disadvantage of the prior art film products is poor resistance to alcohol and oil.
The above-mentioned film materials either applied to wrapping film or applied to packaging bag cannot stand microwave heating in a close pack. The prior art material films rupture easily due to the lack of material strength against the build up pressure inside the bag during the microwave heating. Upon rupture of the film, the build up pressure inside the bag suddenly drops off and moisture escapes immediately thus causing dry, hard and bad taste of foodstuff. The purposes of the prior art films are for one time use only. It is desired to provide a reusable air permeable material film suited for microwave heating. Foodstuffs contained by a packing bag formed of such material film can be well treated with much better moisture preservation, food organoleptic quality and delicacies thereof.
Another prior art composite film as disclosed, by way of example, in U.S. Pat. No. 4,689,936 includes a single venting hole that is formed by removing part of the film. Ordinarily, a so-called die cutting process is performed to remove pre-selected area of the film in order to form the through hole in the composite film. Such prior art method causes weight loss of the film. Further, to prevent situations such as rupture of the film or separation of the film from a food tray during microwave heating, the dimension of the venting hole must be large enough. In this case, a hot melt is necessary to seal the through hole.